The present invention relates to perfluorinated copolymers of tetrafluoroethylene and perfluoro alkyl vinyl ethers. In particular, the present invention relates to copolymers that include a blend of a linear perfluoro alkyl vinyl ether and a branched perfluoro alkyl vinyl ether. The invention further relates to articles produced therefrom and a method of making molded articles using the perfluoro copolymers.
Polytetrafluoroethylene (PTFE) is widely used and is well-known for its excellent mechanical properties combined with chemical inertness, heat resistance, non-flammability, anti-stick properties and exceptional dielectric properties. However, a well-known disadvantage of PTFE is also its extreme high melt viscosity making it unsuitable for melt-processing techniques. As a result, the processing of PTFE into desired shapes or articles requires cumbersome techniques such as sintering.
Accordingly, the art has developed melt-processible perfluoropolymers that can be extruded or molded into shapes and that approach the beneficial properties of PTFE as much as possible. For example, copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) were the first melt processible perfluoropolymers developed. These copolymers, which are known as FEP polymers, however do not have desired heat stability and high temperature properties as may be desired in some applications. A further known type of melt-processible perfluoropolymers is disclosed in U.S. Pat. No. 3,635,926 and includes copolymers of TFE and perfluoropropylvinyl ether (PVE). These copolymers are known as PFA (perfluoroalkoxy polymers) and have improved heat stability and high temperature performance.
WO 97/07147 discloses copolymers of perfluoro ethyl vinyl ether with TFE and U.S. Pat. No. 4,864,006 discloses copolymers of TFE with perfluoro methyl vinyl ether. Still further known copolymers of TFE with a perfluorovinyl ether monomer are disclosed in U.S. Pat. No. 4,029,868 disclosing a terpolymer of TFE, HFP and a perfluorovinyl ether and in U.S. Pat. No. 4,262,101 disclosing a copolymer of TFE and perfluoroalkylvinyl ether optionally in combination with HFP.
EP 75 312 discloses FEP type of polymers that have from 80 to 95.8% by weight of units derived from TFE, 4 to 14% of units derived from HFP and 0.2 to 6% by weight of units derived from a branched perfluoro alkoxy alkyl vinyl ether. The resulting polymers are taught to have a good moldability and good stress crack resistance. However, since such polymers are of the FEP type, they do not have the desired heat stability and performance at high temperature.
U.S. Pat. No. 4,381,387 discloses copolymers of TFE, ethylene, HFP and a bulky comonomer. These polymers are taught to be suitable for wire coatings and making of extruded articles. However, these copolymers are not fully fluorinated and are therefore do not have high chemical and heat resistance.
Copolymers of TFE and perfluoroalkyl vinyl ethers are also used to obtain fluoroelastomers. For example, U.S. Pat. No. 4,920,170 discloses a copolymer that consists of (a) 30 to 80 mol % of TFE, (b) 5 to 60 mol % of perfluoro(lower)alkyl vinyl ether and (c) a perfluoroether of the formula CF2xe2x95x90CFOCF2CFXORf in which X is F or CF3 and Rf is a perfluoro alkyl group, and wherein the sum of (a), (b) and (c) is 100 mol %. Accordingly, the here disclosed copolymers would have at least 20 mol % of perfluorovinyl ether monomer which makes these polymers elastomeric as opposed to thermoplastic and semicrystalline as is the case for PFA type of perfluoropolymers.
The thermal degradation takes place mainly via the thermally unstable end groups formed during the polymerization, i.e. from the end of the chain. The mechanism of this degradation is described in more detail in xe2x80x9cModern Fluoropolymersxe2x80x9d, John Wiley and sons, 1997, p. 223. Thermal degradation produces corrosive gases which considerably impair the quality of the final product by way of metal contamination or the formation of small bubbles, and they can corrode tooling and processing machinery. Naturally, the effect increases as molecular weight falls (lower melt viscosity). Degradation can be substantially suppressed by using postfluorination to convert the thermally unstable end groups into stable CF3 end groups, for example as in U.S. Pat. No. 4,743,658 and DE-A-1 901 872.
Another problem associated with thermoplastic semicrystalline copolymers of TFE is the so called mechanical degradation which is believed to proceed through chain breakage during processing of the copolymer. As a result, the lower molecular weight fraction formed may negatively impact the mechanical properties, in particular flex life and stress crack resistance.
Similarly, low molecular weight fractions may also result from the polymerization method by which the copolymers are produced. For example, aqueous emulsion polymerization is often used to produce the aforementioned polymers which generally leads to the presence of a small fraction of low molecular weight polymer. As a result, these polymers may also have less than desirable mechanical properties such as pressure cycle resistance, flex life and creep rupture strength.
Further PFA type polymers are semicrystalline and include spherolites which reduce the transparency. The level of transparency depends on the size of the spherolites that are present in the polymer. Generally, these spherolites will have an average size of 5 xcexcm to 10 xcexcm or even more. Also, because of the presence of large sized spherolites, the surface of articles produced is somewhat rough and therefore picks up dust. It would thus be desirable to further reduce the dust pick-up.
Despite the many known thermoplastic perfluorocopolymers of TFE, there continues to exist a need to find further perfluorocopolymers of TFE that are melt-processible and that have desirable properties. It is desired to find perfluorocopolymers of TFE that have high temperature resistance, high chemical resistance, easy processing, good mold release and good mechanical properties in particular better flex life in combination with better transparency, and better surface smoothness of articles produced from the polymer. It is further desirable to find perfluoropolymers that are particularly suitable for making hoses and pipes particularly for use in heat exchange applications.
In accordance with the present invention, thermoplastic semicrystalline copolymers of TFE and a mixture of a first and second perfluoro vinyl ether and wherein the copolymers have a melting point of at least 285xc2x0 C., preferably at least 300xc2x0 C. are provided. In particular, the first perfluoro vinyl ether ether is a perfluoro n-alkylvinyl ether having 1 to 4 carbon atoms in the alkyl group and the second perfluoro vinyl ether corresponds to the formula: 
wherein R represents a linear perfluoro alkyl group having 1 to 4 carbon atoms.
Such perfluoropolymer can be produced with a thermal conductivity at 23xc2x0 C. of at least 0.19 W/mK and have particularly suitable properties for use in applications such as hoses and pipes, in particular for heat exchange applications. Beneficial properties of such polymers include a good flex life and good transparency.
The thermoplastic perfluorocopolymers of TFE and the mixture of the first and second perfluoro vinyl ether that have a melting point of at least 285xc2x0 C., preferably at least 300xc2x0 C.
generally have small sized spherolites when made into an article through for example molding or extrusion. Accordingly, in a further aspect, the invention relates to an article of a thermoplastic, semicrystalline fluoropolymer having a melting point of at least 285xc2x0 C. and comprising repeating units derived from tetrafluoroethylene, repeating units derived from a first vinyl ether monomer and repeating units derived from a second vinyl ether monomer, wherein said first vinyl ether is a perfluoro n-alkylvinyl ether having 1 to 4 carbon atoms in the alkyl group and said second vinyl ether is a perfluoro vinyl ether having the formula: 
wherein R represents a linear perfluoro alkyl group having 1 to 4 carbon atoms and wherein said article comprises spherolites having an average diameter of less than 5 xcexcm, preferably 2 xcexcm or less as determined by electron microscopy. These polymers are particularly suitable for preparing articles such as pipes or hoses that have a smooth surface making such surface less prone to picking up dust particles.
Preferred perfluoropolymers of the invention are thermoplastic, semicrystalline perfluoropolymers comprising repeating units derived from tetrafluoroethylene in an amount of 84% to 97.9% by weight, between 2 and 10% by weight of repeating units derived from a first vinyl ether monomer and between 0.1 and 6% by weight of repeating units derived from a second vinyl ether monomer, wherein the first vinyl ether is a perfluoro n-alkylvinyl ether having 1 to 4 carbon atoms in the alkyl group and the second vinyl ether is a perfluoro vinyl ether having the formula: 
wherein R represents a linear perfluoro alkyl group having 1 to 4 carbon atoms.
The preferred perfluoropolymers have desirable properties such as high flex life, improved transparency when made into an article, very smooth inner surface when the article is a tube, and desired thermal conductivity properties.
In a final aspect, the present invention also relates to a method of making a molded article with the perfluoropolymers and to molded articles produced therewith. The perfluoropolymers generally also exhibit desired processing properties such as good mold release.
The perfluoropolymers of the invention are polymers that comprise repeating units derived from TFE and repeating units derived from a mixture of a first and second vinyl ether. The perfluoropolymer generally has a melting point of at least 285xc2x0 C., preferably a melting point of at least 300xc2x0 C. The first vinyl ether comonomer is a perfluoro n-alkylvinyl ether wherein the alkyl group has from 1 to 4 carbon atoms. Examples of such comonomers include perfluoro methylvinyl ether, perfluoro ethylvinyl ether, perfluoro n-propylvinyl ether and perfluoro n-butylvinyl ether. Particularly preferred is perfluoro n-propylvinyl ether.
The second vinyl ether included in the perfluorocopolymer is a perfluoro vinyl ether corresponding to the formula: 
wherein R represents a linear perfluoro alkyl group having 1 to 4 carbon atoms. Examples of the second vinyl ether include those in which R represents methyl, ethyl, n-propyl and n-butyl. The vinyl ether according to formula (I) in which R represents n-propyl is particularly preferred.
Generally, the perfluorocopolymer will include between 84% to 97.9% by weight of repeating units derived from TFE, between 2 and 10% by weight of repeating units derived from the first vinyl ether and between 0.1 and 6% by weight of repeating units derived from the second vinyl ether. According to a particularly preferred embodiment, the amount of the first vinyl ether will be between 2 and 6% by weight and the amount of the second vinyl ether will not exceed 3% by weight. Also, preferably the weight ratio of the first vinyl ether to the second vinyl ether is at least 6:1.
The perfluorocopolymer may include further perfluorinated monomers such as for example hexafluoropropylene but only to the extend they do not impair obtaining the desired properties. Generally, when present, the repeating units derived from additional perfluorinated monomers will be less than 4%.
The perfluoropolymers according to the invention generally have an improved thermal conductivity relative to similar PFA type of perfluorocopolymers known in the art that do not include a mixture of the first and second perfluoro vinyl ether. In particular, perfluoropolymers having a thermal conductivity of 0.19 W/mK can be obtained. Such perfluoropolymers are particularly suitable for making hoses and pipes for heat exchange applications.
Additionally, the perfluoropolymers of the invention allow the production of articles such as hoses and pipes that have a smoother surface without the need for additional nucleation with PTFE as disclosed in U.S. Pat. Nos. 5,473,018 and 5,603,999. Generally, articles can be produced from the perfluoropolymers of the invention that have spherolites with an average diameter of less than 5 xcexcm or even 2 xcexcm or less.
The perfluoro copolymers of this invention can be made by aqueous emulsion polymerization as disclosed in U.S. Pat. No. 3,635,926 and 4,262,101, preferably in the absence of non-aqueous solvent. Other methods employing a non-aqueous medium can also be used such as for example disclosed in U.S. Pat. No. 3,642,742.
The present invention particularly provides a process for preparing fluorinated thermoplastics from tetrafluoroethylene, perfluoro n-propyl vinyl ether and which comprises additionally incorporating perfluoro 2-propoxypropyl vinyl ether into the fluorinated thermoplastic.
For aqueous emulsion polymerization, a broad range of temperatures can be used. Because of heat transfer considerations and the use of thermally activated initiators, higher temperatures are advantageous, such as temperatures in the range of about 50xc2x0-100xc2x0 C.
Surfactants generally suitable for use in emulsion polymerization of TFE copolymers can be used. Such surfactants include, for example, ammonium perfluorooctanoate (C-8), ammonium perfluorononanoate (C-9), and the perfluoroalkyl ethane sulfonic acids and salts thereof disclosed in U.S. Pat. No. 4,380,618.
Initiators that can be employed in the emulsion polymerization of the perfluoro copolymers are water-soluble free-radical initiators such as ammonium persulfate (APS), potassium persulfate (KPS), or disuccinic acid peroxide, or redox systems such as those based on potassium permanganate. APS and/or KPS is preferred.
Chain transfer agent (CTA) can also be used in aqueous polymerization of the perfluoro copolymers of this invention, and use of CTA is preferred. A wide range of compounds can be used as CTA. Such compounds include, for example, hydrogen-containing compounds such as molecular hydrogen, the lower alkanes, and lower alkanes substituted with halogen atoms. The chain transfer activity of such compounds when used can result in copolymer having xe2x80x94CF2H end groups which are relatively stable. The CTA can contribute other relatively stable end groups, depending on the identity of the CTA. Preferred CTAs include methane, ethane, and substituted hydrocarbons such as methyl chloride, methylene chloride and chloroform. The amount of CTA used to achieve desired molecular weight will depend, for given polymerization conditions, on the amount of initiator used and on the chain transfer efficiency of the chosen CTA. Chain transfer efficiency can vary substantially from compound to compound, and varies with temperature.
The thermal stability of the perfluoro polymers of the invention can be further improved by converting any thermally unstable end groups into stable xe2x80x94CF3 end groups, through fluorination of the agglomerate or granulated melt. The resulting perfluoro polymers preferably have fewer than 70 unstable end groups per 106 carbon atoms. This can reduce corrosion of processing machinery and reduce metal contamination of the products produced from the perfluoro polymers.
The perfluoro polymers of the invention can be melt processed including for example by extrusion and molding techniques such as injection molding and transfer molding. Particularly when processed via molding, beneficial properties of the perfluoro polymers were found. For example in transfer molding for lining ball-valve bodies, the perfluoropolymers of the invention were found to perform better than prior art perfluorocopolymers of the PFA type. In particular the perfluoro polymers of the invention can be demolded very easily under conditions in which prior art PFA type of polymers are generally difficult to demold. This improvement gives marked advantages in processing, since a considerable portion of faulty production in transfer molding is attributable to poor demoldability.
The invention will now be further illustrated by reference to the following examples without however the intention to limit the invention thereto.